Composting Leather By-products
By E. Dring
Hides and skins are a by-product of the meat industry. Raw hides and skins biodegrade rapidly in compost (<10 days); the addition of chemicals (during tanning) can alter the rate of disintegration. The tanning process results in chemistry which needs to be controlled to reduce the leather’s overall environmental risk (Sundar et al., 2011).
Different methods can be used to decompose leather by-products. Depending on the chemistry of leather, some leathers are observed to decompose at a faster rate. In a study measuring the anaerobic biodegradation of leather offcuts, vegetable-tanned leather was over three times more biodegradable than chrome-tanned leather and over nine times more biodegradable than synthetic materials (Pantazi and Vasilescu, 2014).
A composting windrow method can be used to treat tannery sludge. Tannery sludge can be aerobically composted with crushed wood (3-part wood: 1-part sludge). The process dries out the sludge, in turn, reducing the weight of the waste product by approximately 70%. Most of the wood can be reused in the next process (Framis et al., 2018). Positive effects of composting tannery sludge include the pasteurisation of pathogen by high temperatures and antibiotics from actinomycetes. As a result, a 99% decrease in E. coli has been found after a windrow composting process (Akinci et al., 2013).
The co-composting of animal by-products with lignocellulosic waste can produce a stabilised matter (Cayuela et al., 2012). For example, tannery sludge can be aerobically composted with rice bran and cow manure. The compost produced was found to have a high bean germination index. Therefore, Lambu et al. (2019) suggested that this process could remove some phytotoxic compounds from the organic waste.
Vermicomposting has been seen to produce compost rich in nutrients from animal fleshing waste. Tomato plants were observed to grow from the compost with a germination index of 84% (Ravindran et al., 2008).
Impacts on Soil Application:
Compost containing leather can have positive effects on plant growth if the leather chemistry contains high levels of nutrients. However, leather can also have negative effects on plants, such as stunted growth and surface rooting (Figure 1). Compost from waste needs to pass health and sanitary checks before being applied to agricultural land. The microorganisms and elements of the soil should not be a health hazard to humans or animals by touch, digestion of crops, or water and air pollution. A concern with compost from waste is the concentration of heavy metals. Some of which are important for plant growth in small proportions (Krstic et al., 2019). Metals such as copper and zinc are important nutrients for metabolic reactions in plants and animals (Onbasi et al., 2018). Excess copper on the other hand can lead to reactive molecules and subsequent cell damage to the plants they are in contact with (Rehman et al., 2019).
Figure 1 - Compost containing leather can have positive (enhancement) or negative (suppression) effects on plant growth.
When composted, if tannery sludge is applied to soil in small quantities (2.5 to 5 Mg/ha), the organic matter content of the land increases, thus, enhancing soil fertility (Sousa et al., 2017). However, the soil should be monitored for metal accumulation, such as chromium and calcium. These elements can alter the soil's pH and have negative impacts on soil microorganisms, in turn, reducing the available nutrients for plant growth.
Plants take up soluble chromium (III), a micronutrient important for plant health. Chromium (VI) on the other hand is toxic – it is a powerful oxidising agent (Ertani et al., 2017). Nunes et al. (2018) studied the effect of vermicompost with the addition of tannery waste on the growth of sweet peppers. It was found that the addition of tannery waste containing chromium did not significantly change the concentration of chromium (III) in the fruits of plants, and chromium (VI) was not identified. The concentrations of chromium in the food were suitable for human consumption.
Composted leather can unfortunately have negative effects on plant growth. A bioassay method was used to observe the effects of compost tannery sludge on lettuce (Lactuca sativa). Germination was not significantly affected, however, solid samples resulted in abnormal root growth. An onion (Allium cepa) bioassay suggested that compost tannery sludge reduced cell viability and increased mutations (Gonçalves et al., 2020). Consequently, plants may have a lower yield.
Composting waste from the leather industry can reduce the volume of waste entering landfill sites. Compost containing decomposed leather could be applied to agricultural land in the future for soil remediation. Current issues involve the accumulation of elements having negative effects on plant growth and toxicity. Diluting the concentration of leather waste in compost or reducing toxic chemicals in the leather process could make leather compost safe for agricultural purposes.
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